The functionality of the mechanical filter has been replicated in electronic oscillators for more than 100 years. Recent interest in mechanical oscillators has been motivated by the fact that quality factor (frequency selectivity and noise threshold) of mechanical oscillators can be in excess of 106 while small electronic oscillators have Q=5-10, and bulky expensive ones top out at Q=100.
It is also well known that applying force to the base of a clamped beam causes its stiffness to drop, until the structure eventually buckles into a higher order mode shape. There is a rich field using this technology for vibration protection and isolation systems, but in a fully passive/static manner. The basics are detailed by Alabuzhev (published book, “Vibration protecting and measuring systems with quasi-zero-stiffness”).
It is also common to use a mechanical system as a filter/amplifier. An excellent review of this field is the article by Nguyen, entitled “MEMS Technology for Timing and Frequency Control,” IEEE Trans., Vol 54, Num 2. All current filters include some high-quality resonating portion, a driving portion, and a sensing portion. FIGS. 1A-1E depict several examples from this reference. None of them are tunable. Shown in FIG. 1A-1E are a Clamped-Clamped Beam 100a, a Free-Free Beam 100b, a Wine-Glass Disk 100c, a Countour-Mode Disk 100d, and a Spoke-Supported Ring 100e, respectively.
If one wishes to create a broadband mechanical oscillator, the current state of the art is to create an array of oscillators. This has limitations, in that one resonator needs to be manufactured for each frequency. A good example of this is in U.S. published patent application 20130207746 A1, which describes an array 200 of square-plate oscillators, shown in FIG. 2.
Optical resonators, however, may be tuned by altering the geometry of their resonator. For example, the article by Pöllinger, M. et al. entitled “Ultrahigh-Q Tunable Whispering-Gallery-Mode Microresonator” describes a tunable optical resonator 300, which uses a piezoelectric cantilever 310 to vary the tension in an optical fiber 320, as shown in FIG. 3A. FIG. 3B is a plot illustrating an example of the relative resonance frequency versus applied voltage Upiezo for the resonator 300 of FIG. 3A.
It is also possible to tune the resonant frequency of a mechanical oscillator by changing its elastic constant, via a phase change or field coupling. A classic example more than 50 years old is the YIG filter, as disclosed in U.S. Pat. No. 3,435,385, which uses a magnetic field to tune the resonance of a ferroelectric sphere. YIG filters are orders of magnitude larger than practical for some applications, are expensive, and power hungry. Another example is tuning the resonator via an electric field applied to an integrated piezoelectric substrate, as shown in U.S. Pat. No. 6,943,484 B2. The frequency range for this method is low and the piezoelectric can spoil other properties of the resonator.
Another very common example is tuning via capacitive “virtual” springs, as described in U.S. Pat. No. 8,450,913 B1. These work well, but only in a very narrow range (0.01% frequency change) as capacitive springs are very weak and nonlinear at even moderate displacements. They are only used for “trimming” small defects and our invention surpasses their range by at least 1000×.
There is also prior art for coupling the oscillator to a physical body and amplifying mechanical signals, i.e. a single frequency from a microphone diaphragm. This is the principle that many biological organisms use for hearing, using a chemical “drive circuit” on a mechanical amplifier (typically hair bundles) to detect a single sound frequency. These structures even have built-in frequency tuning, but they use chemical/biological mechanisms which cannot be transitioned to integrated circuits. (Hudspeth, “Making an effort to listen: mechanical amplification in the ear,” Neuron, 2008) The amplifier concept has been applied to a physical microphone by Reichenbach, in “Unidirectional Mechanical Amplification as a Design Principle for an Active Microphone” but does not possess the desired tunability.
What is needed is a single resonator that functions across a continuous bandwidth rather than discrete channels. Moreover, what is needed is a tunable resonator mechanism allowing a high degree of tuning. Furthermore, what is needed is a mechanical, rather than optical resonator.